The present invention relates generally to the field of laser diodes, and more particularly to short-wavelength nitride based laser diodes. Short-wavelength nitride based laser diodes provide smaller spot size and a better depth of focus than red and infrared (IR) laser diodes for laser printing operations and other applications. Single-spot nitride laser diodes have applications in areas such as high density-optical storage.
Laser diodes based on higher bandgap semiconductor alloys such as AlGaInN have been developed. Excellent semiconductor laser characteristics have been established in the near-UV to violet spectrum, principally by Nichia Chemical Company of Japan. See for example, S. Nakamura et al., xe2x80x9cCW Operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substratesxe2x80x9d, Applied Physics Letters, Vol. 72(6), 2014 (1998), S. Nakamura and G. Fasol, xe2x80x9cThe Blue Laser Diode-GaN based Light Emitters and Lasersxe2x80x9d, (Springer-Verlag, 1997) and also A. Kuramata et al., xe2x80x9cRoom-temperature CW operation of InGaN Laser Diodes with a Vertical Conducting Structure on SiC Substratexe2x80x9d, Japanese Journal of Applied Physics, Vol. 37, L1373 (1998), all of which are incorporated by reference in their entirety.
For laser diodes and arrays incorporated into printing systems, reliable, low threshold operation is a basic requirement. Among the difficulties associated with achieving low threshold operation is the confinement of injected electrons in the quantum well active region. If the injected electrons are not appropriately confined, the electrons leak away from the quantum well active region and recombine with the holes injected from the p-layers of the structure. For example, in the nitride laser structure pioneered by S. Nakamura of Nichia Chemical Company, a thin, high bandgap layer is placed immediately adjacent to the active region to confine the injected electrons. In the Nakamura structure, a 200 xc3x85 layer of Al0.2Ga0.8N:Mg acts as a tunnel barrier layer to prevent the energetic electrons (electrons having sufficient energy to escape from the quantum wells) from diffusing into the p-type material, where recombination with the available holes would occur. Reducing electron leakage lowers the laser threshold current and its temperature sensitivity while raising the quantum efficiency of the laser.
FIG. 1 shows conventional nitride laser structure 100. Conventional nitride laser structure 100 uses both GaN:Mg p-waveguide layer 115 and GaN:Si n-waveguide layer 116 with Al0.2Ga0.8N:Mg tunnel barrier layer 110 positioned over In0.12Ga0.88N/In0.02Ga0.98N:Si multiple quantum well active region 120. Al0.07Ga0.93N:Mg p-cladding layer 130 is positioned over p-waveguide layer 115 while Al0.07Ga0.93N:Si n-cladding layer 131 is positioned below n-waveguide layer 116. GaN:Mg layer 140 serves as a capping layer to facilitate ohmic contact while Al2O3 layer 150 serves as the growth substrate. An Ni/Au p-contact (not shown) on top of GaN:Mg layer 140, a Ti/Al contact (not shown) on exposed surface of GaN:Si layer 155. GaN:Si layer 155 is a lateral contact layer while In0.03Ga0.97N:Si layer 156 is the defect reduction layer to prevent defect propagation. GaN layer 160 functions as a buffer layer.
FIG. 2 illustrates the function of tunnel barrier layer 110 using a simplified band diagram. Tunnel barrier layer 110 is a p-type AlGaN layer which acts as a strong confinement barrier for injected electrons. Quantum wells 220, 221, 222, 223 and 224 comprising active region 120 are InGaN while tunnel barrier layer 110 is AlGaN. The potential energy level 250 for the conduction band electrons and quasi-fermi level 255 are shown for AlGaN tunnel barrier layer 110 with low p-doping energy level 230 and high p-doping energy level 240 are shown with respect to potential energy level 250 for electrons and fermi level 255 for the conduction band. Quasi-fermi level 260 for the holes is shown along with potential energy level 265 for holes. Successful operation of Nakamura type laser structures requires successful p-type doping of high-bandgap AlGaN tunnel barrier layer 110. However, the growth of tunnel barrier layer 110 presents many practical difficulties, including the difficulty of p-doping high aluminum content alloys and the difficulty of reliably growing high aluminum content alloys because of parasitic pre-reactions between trimethylaluminum and ammonia during metalorganic chemical vapor deposition (MOCVD). If the hole concentration or aluminum content in tunnel barrier layer 110 is insufficient, the ability of layer 110 to contain electrons is reduced since electron confinement increases with the p-type doping level.
P-cladding layer 130 can be used to confine injected electrons in a nitride laser diodes if it is placed in close proximity, typically within 1 minority carrier diffusion length, to the multiple-quantum well active region. A difficulty with this approach is that multiple-quantum well active region 120 is typically located at the core of a waveguide region to maximize the spatial overlap with the optical mode as shown in FIG. 3 for conventional nitride laser diode structure 100. However, this places p-cladding layer more than 1 minority carrier diffusion length from multiple-quantum well region 120. Refractive index profile 310 and corresponding fundamental transverse optical mode 320 are shown as a function of distance relative to the interface between n-cladding layer 131 and n-waveguide layer 116. The waveguide thickness is adjusted independently to maximize the optical confinement factor, ┌. Optical confinement factor, ┌ is the fraction of the power that overlaps multiple-quantum well active region 120 where the optical gain is generated. For nitride laser diodes, the typical thickness for the waveguide above and below multiple-quantum well active region 120 is about 100 nm which is greater than 1 electron diffusion length. This places p-cladding layer 130 in conventional nitride laser diode structure 100 to far away from multiple-quantum well active region 120 to confine the injected electrons.
In accordance with the present invention, a p-type cladding layer is used to eliminate the p-type waveguide and eliminate the need for a p-type, very high bandgap, high-aluminum content AlGaN tunnel barrier layer in nitride laser diodes. The p-type cladding layer is used to suppress electron leakage. In addition to the p-type cladding layer, a high-Al-content tunnel barrier, a superlattice structure or a distributed electron reflector may be placed at the multiple quantum well region/p-cladding layer interface. Although a p-type cladding layer is used for suppressing electron leakage in laser diodes fabricated from other materials such as arsenides and phosphides, the use of p-cladding layer in nitride laser diodes is not straightforward. The minority carrier diffusion lengths (average distance minority carrier travels before recombination occurs) in nitrides are many times shorter than in other laser diode materials. Hence, the p-cladding layer typically lies several diffusion lengths away from the multiple-quantum well active region. Consequently, injected electrons are not appreciably confined by the p-cladding layer which leads to the requirement for the high-aluminum content tunnel barrier layer. In red and infrared laser diodes, the waveguide thickness is a mere fraction of the diffusion length, so that the cladding layer can effectively suppress leakage, see for example, xe2x80x9cDrift leakage current in AlGaInP quantum well laser diodes,xe2x80x9d D. P. Bour, D. W. Treat, R. L. Thomton, R. S. Geels, and D. F. Welch, IEEE Journal of Quantum Electronics, vol. 29, pp. 1337-1343 (1993).
A high optical confinement factor can still be achieved for nitride laser diode structures if a p-cladding layer is positioned adjacent to the multiple-quantum well active region instead of the typical 100 nm distance away which maximizes the optical confinement factor. This is due to the relatively weak transverse (perpendicular to the layer planes) waveguiding that occurs in nitride lasers which results in much of the mode spreading evanescently into the cladding layers. Indeed, the refractive index difference, An, between the waveguide core and the cladding layers is only about 0.05 which is nearly one order of magnitude less than that in typical AlGaAs lasers. The weak transverse waveguiding results in a less strongly peaked waveguide mode which makes the optical confinement factor less sensitive to any wave guide asymmetry.
A superlattice may be introduced into the asymmetric waveguide nitride laser diode structure or a conventional nitride laser structure to enhance carrier confinement. The superlattice is used to replace a uniform bulk layer. A properly designed superlattice inhibits the tendency for structural defect formation while allowing adequate p-type doping and carrier confinement in the quantum wells. For example, a superlattice that alternates GaN with AlGaN layers allows high p-doping since the GaN layers are readily p-doped. Carrier confinement requires adequate band offsets in the valence and conduction bands between the quantum well active region and the surrounding layers. Carrier confinement by superlattice structures also requires avoiding resonant tunneling effects.
Short period superlattices may be designed to act as coherent electron reflectors. Short period superlattices function as distributed Bragg reflectors which reflect the wavefunction of leaked electrons back into the multiple quantum well active region. Similar structures, often called xe2x80x9cMulti-Quantum Barriersxe2x80x9d are used to confine electrons in short wavelength (xcex less than 650 nm) red AlGaNInP lasers where they are placed in the p-cladding layer rather than immediately next to the multiple quantum well active region. As coherent reflections may be produced using low-bandgap superlattice layers, the need for AlGaN layers may be reduced or eliminated. This preserves the structural quality of films while transverse waveguiding is not negatively effected by AlGaN layers and p-type doping benefits from the ability to use low-bandgap barrier layers. The thickness of the layers making up the superlattices needs to be selected to avoid resonant tunneling. Appropriate selection of layer thicknesses allows an electron reflectivity of about 100% for electron energies beyond the classical barrier height. Therefore, properly designed distributed electron reflectors may be more effective than bulk barrier layers for confining injected electrons.
Hence, in accordance with the present invention, nitride laser diode structures can be made which eliminate the need for the p-type waveguide layer and the high-aluminum-content tunnel barrier and have a p-cladding layer deposited above the multiple quantum well active region to confine electrons. Additionally, superlattices may be introduced between the multiple quantum well region and the p-cladding layer to enhance carrier confinement.